Composite mist pass after-the-fact to finished rendered video using the original blend
I think the way to do this is as you said: low bounces, no caustics, etc.

You can bring the already rendered image sequences into the compositor with the image node:


Add an image node (ShiftA> Input > Image)
Open the first frame of your image sequence in the image node
Change the source type to Image Sequence:


Set the Frames, Offset and Start frame values.  


Frames is the number of frames to use.
Start frame is the scene frame on which to display the first frame
Offset is the amount to offset the current frame. (e.g. with Offset = 5, the image for frame 6 will be read instead of the image for frame 1)



You can load your sequence of Multilayer images alongside your original render into the compositor in this fashion. If you rendered the Multilayer images with the render passes enabled you should be able to use the passes from the Image node.

You can also use this alongside the Renderlayer node to composite with the render directly, e.g. with a 150 frame image sequence: